It uses cilantro roots (coriander roots and stem) - which typically a Chinese home cook will discard, using only the coriander leaves. Waste not, want not, once you start cooking Thai dishes you will begin to appreciate the the flavours and texture from the roots and stems. 

You combine it with white peppercorn and garlic. Pound it, adding fish sauce along the way and you will have a base for marinade which is behind many Thai grill recipes.    

My go to cookbook for Thai dishes is Chef Mcdang's The Principles of Thai Cookery. I deeply cherish a cookbook which is written in such a way that it helps you to appreciate the principles behind a cuisine. I have tried and loved some recipes from there. 

I adapted one of his recipes for grilled chicken and simplified it for my own dinner table. Since I started cooking it, it has become a regular feature. The fish sauce marinated skin of a boneless thigh crisps up very well and all it takes is 20 minutes in the oven on 180C. 

Here is the recipe.

To feed 4-6 pax

Marinade:

15 pieces of white peppercorn. 

3 pieces of coriander root and stems

20 pieces of small Thai garlic 

1 inch of fresh turmeric

3 tablespoon of fish sauce

1 teaspoon of salt 

Dipping Sauce:

Tamarind Sauce

Thai Palm sugar or white sugar

Fish Sauce

Coriander stems

1. Pound the turmeric, coriander roots and garlic into a paste, adding fish sauce along the way. 
2. Rub the paste into the chicken and marinade for 2 hours or more. 
3. Pre-heat the oven at 180C. 
4. When ready, place the chicken skin side up and roast for 20 minutes or till the skin is nicely crisped and browned. 
5. For serving, slice the chicken. Use a chopper to chop it cleanly so that the crisped skin stay with the meat. 
6. Serve it with fresh coriander leaves and the tamarind sauce. 

Making the Sauce: Seasoned the tamarind sauce with fish sauce and sugar to get a balance of sour, salty and sweet. Chop some coriander stems and add to the sauce.

Notes:

1. Use a superior fish sauce brand which is less salty. My favourite is the Thai brand Masterchef or the Vietnam ones, which are usually less salty. Normally the better fish sauces are sold in glass bottles, not plastic. 
2. You can buy small Thai garlic in the Thai super mart at Golden Mile. I prefer to use the smaller and more pungent Thai garlic but you can easily replace it with you regular garlic. 
3. You can also use a whole or half chicken but using boneless meat just makes it faster and easier. You can use the same marinade to roast a whole chicken.

After 20 minutes in the oven